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Wavelength vs Wavenumber etiquetteWavenumber is just a convenience -- it's a lot easier to write $e^{ikx}$ than it is to write $e^{2\pi i x\frac\lambda}$. And if you're writing the first thing, you need a name with which to refer to $k$. They are 100% the same thing, though.

2d

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Why is SiO2 solid while CO2 is a gas?Considering that the sublimation point of dry ice is only like $-70{}^{\circ}$ C, a factor of 2.5 of the mass is actually probably plenty to explain this.

Density of dark matter along the galaxy@RussellMcMahon: of course, modified gravity may still be right, but you're asking way more of Occam with most realistic modified gravity schemes (seriously, look up the TeVeS Lagrangian) than you are by just saying "let's extend the standard model to include a stable 20 TeV [or whatever] WIMP plus a one-parameter cosmological constant". YMMV, of course, but non-radiative matter isn't a wild assumption.

Is momentum an invariant?Like, the quantity you're talking about is explicitly frame-dependent, as your notation makes manifest. It's not an invariant.

Jul27

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Is momentum an invariant?@user12262: the frame velocity is 100% not Lorentz covariant. It is explicitly present in the equation. Therefore, a boost will change the frame velocity, and change your momentum.

Why we don't see time dilation in stars orbiting black hole?@eli.rodriguez: also note what I said in my answer when talking about accretion disks. Largely, they are still in orbits until they do the final plunge, but the final plunge is very fast, so the bulk of the luminosity of the disk will come from the plasma orbiting at a radius greater than $6M$ for a Schwarzschild hole.

When is Einstein summation implied by Lorentz indices?I also can't think of a case where anyone would take the sum $\sum_{\mu}\sum_{\nu}A^{\mu \nu}$. Almost, universally, they are taking the trace of the matrix, which is labeled ${\rm Tr}(A^{\mu \nu})$ or they are contracting it on the metric, $g_{\mu \nu}A^{\mu \nu}$. The latter thing would usually just be called $A$